I can configure my camera (5D) to use long exposure noise reduction (dark frame is exposed and subtracted), but is this method really effective when shooting RAW? Or is the actual subtraction only done when shooting JPEG?

5 Answers
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It's effective. Basically, the camera can use the second, dark frame, to subtract signal out of the first frame, so it definitely gets used in RAW. In fact, if you try it, you'll discover that you end up with only one RAW image as the DFS image is discarded after use.

On this topic, however, I'd note that you want to use this carefully. If you're doing long night exposures, DFS can double the amount of time it takes to snap a shot and that cuts into your shooting time. Many astrophotographers will do their own DFS after the fact as a result of that and us Pentaxians are gnashing our teeth because we have no option to shut it off (my only complaint about Pentax).

+1: Great point about astrophotography...it really hangs you out to dry when your trying to get nice sky shots on those rare, fully dark nights.
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jrista♦Aug 21 '10 at 0:03

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@jrista I got bit by it once doing by doing a one hour exposure trying for star trails... Fortunately my battery was fully charged.
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John Cavan♦Aug 21 '10 at 0:46

@John, what happens if you run out of battery during the dark-frame? Does it still write the original exposure?
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EruditassAug 21 '10 at 1:39

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Not sure if its the same on the Pentax, but if I just switch my Canon off during dark-frame capture I'm pretty sure (its been a long time since I've left dark-frame subtraction on ;) it writes the file to the flash (presumably ditching the dark frame subtraction). So could be worth trying it out to see if you can manually skip the dark frame on particular shots?
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drfrogsplatAug 22 '10 at 2:11

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I have found that exposures longer than 1 min (without the dark frame subtraction) with my 5D & 5DII exhibit noise that sort of looks like grains of rice. Using a variety of noise reduction software I've found them difficult to eliminate. The dark frame subtraction seems to work much better. Interestingly my old 20D is much cleaner with long, night time exposures, even with the NR turned off.
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Henry PeachAug 23 '10 at 13:24

Yes, the long exposure noise reduction is applied to the RAW file also.

Regular noise reduction is not done on the RAW data. As that only uses the data in the image, it can be repeated afterwards. The long exposure noise reduction uses the data from another exposure to correct the data, so that can't be repeated from only the RAW data.

Can you test it out? Take a picture with noise reduction off, then with noise reduction on, and then you'll see.

As you said, it's probably a dark frame subtraction, meaning that any bias patterns that are present in the chip will get removed. These bias patterns show up on longer exposures, as in, some pixels may be more sensitive than others, so the dark frame will identify those inter-pixel biases and allow the camera to correct them. That means that it should affect RAW frames, because the intensity information from the dark frame will still be removed.

I know that it worked on my D70 back when I took pictures of the night sky. It's harder for me to find a region without light pollution nowadays.

Wasn't really intended this way (and I didn't have this information when I asked the question), but my experience shows different results. I made a 556-second exposure at ISO 400 with lens cap on and long exposure noise reduction switched on and RAW+JPEG configured. The results are 100% crops with no additional processing applied.

In-camera JPEG:

Canon DPP (noise reduction disabled):

Adobe Camera RAW (noise reduction disabled):

Adobe Camera RAW + Topaz Denoise (RAW-moderate setting):

My conclusion is that there is no point to shoot only RAW with long exposure noise reduction on (expose and subtract dark frame) as it doubles exposure time and it's effect is questionable unless done by camera.

Its very hard to see...perhaps a long shot of something around 18% gray would be better?
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jrista♦Aug 21 '10 at 23:53

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JPEG has additional processing applied which, I think, you haven't matched. One obvious one is noise reduction, other than with Topaz, but you cannot assume that Topaz is doing the same thing as the camera. In fact, Topaz has created a more even appearance which I would expect from a black cap shot. Also, DFS isn't perfect, heat effects noise and the second exposure will be hotter than the first.
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John Cavan♦Aug 22 '10 at 2:23

@jrista - well, on my laptop monitor they're indeed all black although there was a big difference on my photo editing monitor :) I'll boost brightness in even amounts once I get to it. @John - I got what I wanted to know - there's no point to shoot only RAW with long exposure noise reduction turned on and waste double time. I don't want to start another argument. You can edit my answer if you feel that it's somehow not fair.
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KarelAug 22 '10 at 8:17

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Do you have a 'control' set, with the same exposure time and post-processing techniques, but with long exposure noise reduction turned off?
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Evan KrallAug 26 '10 at 20:54

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@Evan - good question, just it's kinda hard to find any motivation to work on it when the speculative answers rule.
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KarelAug 27 '10 at 8:24

I've found that when using a Tv between 1 and 30 seconds or so it is quite effective. Once I process in DPP I am able to use less NR and save more detail. The subtraction is done to the RAW file, but that doesn't mean all the noise will be eliminated, only that caused by bias patterns in the chip. Other sources of noise, such as shot noise, will still require NR in post.

I don't have the patience to try it at longer than 30 second exposures.